The present invention relates to a method of heat-treating a nitride compound semiconductor layer and a method of producing a semiconductor device.
In recent years, gallium nitride compound semiconductors such as GaN, an AlGaN compound crystal and an AlInGaN compound crystal are considered promising as component materials for semiconductor devices that can emit light in the range of from a visible light region to an ultraviolet light region. Particularly, since a light emitting diode (LED) using a gallium nitride compound semiconductor has come to be commercially used, the gallium nitride compound semiconductor devices have been of great interest. Further, a semiconductor laser (laser diode, LD) using a gallium nitride compound semiconductor has been reportedly realized as well, and applications thereof including a light source for an optical disk are expected.
When a gallium nitride compound semiconductor layer doped with a p-type impurity is formed by a vapor deposition method, the gallium nitride compound semiconductor layer formed by such a method does not become a p-type, but it constitutes a semi-insulating layer having a high resistance of at least 108 Ω·cm or more, i.e., an i-type compound semiconductor layer.
There is known a method of decreasing the resistance of the above i-type compound semiconductor layer having a high resistance to convert it to a p-type compound semiconductor layer, as is disclosed, for example, in JP-A-2-257679. In the method disclosed in JP-A-2-257679, the surface of an i-type gallium nitride compound semiconductor layer obtained by doping the layer with Mg as a p-type impurity is irradiated with electron beam to decrease the resistance of the surface of the gallium nitride compound semiconductor layer. In the above method, however, it is only of the surface of the gallium nitride compound semiconductor layer that the resistance can be decreased, and the above treatment takes a long time due to scanning with electron beam. Further, there is involved a problem that it is difficult to uniformly decrease the resistance of the gallium nitride compound semiconductor layer in a wafer plane.
Published Japanese Patent No. 2540791 discloses a technique of growing a gallium nitride compound semiconductor doped with a p-type impurity by a vapor deposition method, and heat-treating the semiconductor at a temperature of 400° C. or higher or at a temperature of 600° C. or higher for attaining a practical carrier concentration. The above heat-treatment is carried out in vacuum or an inert gas atmosphere free of hydrogen atoms of NH3 or H2 for approximately 10 to 20 minutes.
In production of a semiconductor laser (LD), however, diffusion of a p-type impurity such as Mg is presumably more liable to take place, or the sharpness of an interface in a superlattice structure is presumably more liable to be disintegrated by the diffusion of In with an increase in the heat treatment temperature, and the deterioration of an active layer such as an increase in threshold voltage Ith and a decrease in lifetime is liable to proceed easily.
Further, when the heat treatment is carried out at a high temperature, deterioration takes place in the surface of the gallium nitride compound semiconductor layer due to dissociation of nitrogen atoms. For preventing the above phenomenon, Published Japanese Patent No. 2540791 also discloses a technique for forming a cap layer on the surface of a gallium nitride compound semiconductor layer. However, the material for constituting the cap layer is selected from GaxAl1-xN (0≦x≦1), AlN, Si3N4 or SiO2, and finally, it is required to remove the cap layer made of such a material from the surface of the gallium nitride compound semiconductor layer, which results in a problem that the number of manufacturing steps increases.